* Optimized list appends and pops by making fewer calls the underlying system

  realloc().  This is achieved by tracking the overallocation size in a new
  field and using that information to skip calls to realloc() whenever
  possible.

* Simplified and tightened the amount of overallocation.  For larger lists,
  this overallocates by 1/8th (compared to the previous scheme which ranged
  between 1/4th to 1/32nd over-allocation).  For smaller lists (n<6), the
  maximum overallocation is one byte (formerly it could be upto eight bytes).
  This saves memory in applications with large numbers of small lists.

* Eliminated the NRESIZE macro in favor of a new, static list_resize function
  that encapsulates the resizing logic.  Coverting this back to macro would
  give a small (under 1%) speed-up.  This was too small to warrant the loss
  of readability, maintainability, and de-coupling.

* Some functions using NRESIZE had grown unnecessarily complex in their
  efforts to bend to the macro's calling pattern.  With the new list_resize
  function in place, those other functions could be simplified.  That is
  being saved for a separate patch.

* The ob_item==NULL check could be eliminated from the new list_resize
  function.  This would entail finding each piece of code that sets ob_item
  to NULL and adding a new line to invalidate the overallocation tracking
  field.  Rather than impose a new requirement on other pieces of list code,
  it was preferred to leave the NULL check in place and retain the benefits
  of decoupling, maintainability and information hiding (only PyList_New()
  and list_sort() need to know about the new field).  This approach also
  reduces the odds of breaking an extension module.

(Collaborative effort by Raymond Hettinger, Hye-Shik Chang, Tim Peters,
 and Armin Rigo.)

1 files changed, 11 insertions, 0 deletions

diff --git a/Misc/NEWS b/Misc/NEWS
index d9778a3..275593b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,17 @@ What's New in Python 2.4 alpha 1?
 Core and builtins
 -----------------
 
+- Optimized list resize operations to make fewer calls to the system
+  realloc().  Significantly speeds up list appends, list pops,
+  list comprehensions, and the list contructor (when the input iterable
+  length is not known).
+
+- Changed the internal list over-allocation scheme.  For larger lists,
+  overallocation ranged between 3% and 25%.  Now, it is a constant 12%.
+  For smaller lists (n<=5), overallocation was upto eight bytes.  Now,
+  the overallocation is no more than one byte -- this improves space
+  utilization for applications that have large numbers of small lists.
+
 - Support for arbitrary objects supporting the read-only buffer
   interface as the co_code field of code objects (something that was
   only possible to create from C code) has been removed.